Synthetic fiber preparation



Patented Oct. 20, 1953 George E. Ham, Decatur, Ala., assignor to The i IChemstrand Corporation, Decatur, Ala a corporation of Delaware NoDrawing. Application October 15,1952,

3' 1 This invention relates to a new method preparing synthetic fibersfrom copolymers of acrylonitrile. More particularly the inventionrelates to anew solvent for acrylonitrile polymers, and to completelymiscible mixtures including acrylonitrile copolym'ers, from whichmixtures quality-fibers can be extruded.

This application is a continuation-in-part of my copending applicationSerial No. 117,506, filed September 23, 1949, for Synthetic FiberPreparation, now abandoned It is well-known that copolymers of over '75percent acrylonitrile are capable of being fabricated into'hi-ghstrengthfibers. The conventional technique for preparing fibers fromthese copolymers involves the dissolution of the copolymer in a suitablesolvent and thereafter extruding the viscous solution sopreparedthroughan orifice into a medium which'removes the solvent and precipitates theacrylonitrile copolymer ina continuous form. Many solvents have beenproposed but many of them are impracticable due to the cost and due tothetendency of the solution to gel upon standing, especially at elevatedtemperatures encountered in the'spinning operation.

The primary purpose of this invention is to provide a new low costsolvent for the prepare.- tion of synthetic fibers. A further purpose ofthis'invention is toprovide a solvent which forms more stable mixturesor solutions of acrylonitrile copolymers. A still further purposeof theinvention is to provide amethod of forming fibers of high tensilestrength and desirable elongation. It has been found that mixtures ofdiethyl phosphite and N,N-dimethylacetamide or N,N- dimethylformamide'are excellent solvents for acrylonitrile copolymers. Not all copolymersof acrylonitrile are soluble in all mixtures of diethyl phosphite andthesaid N,N-dimethylamides. It has been found, however, that there is adefinite relationship between proportions of diethyl phosphite and the'N,N-dimethylformamide or N,N'-dimethylacetamide and the quantity ofacrylonitrile in the copolymer. However, it has been found that solventmixtures containing more than percent of diethyl phosphite will notdissolve polyacrylonitrile and copolymers of more than a fixed maximumamount of acrylonitrile. The maximum acrylonitrile content may bedefined by the relationship y=90+0.14:c, wherein a: is the percent ofthe N,N-dlmethylamide and y is the maximum acrylonitrile proportion inthe copolymer. In addition, it has been further found that polymerscontaining Serial No. 314,951 6 Claims. (01. 260- -30.6)

more than 90 percent by weight of acrylonitrile are insoluble in purediethyl phosphite while polymers containing 90 percent or less ofacrylonitrile arev soluble in pure diethyl phosphite; In order'todissolve the polymers containing more than 90 percent by weight ofacrylonitrile, it-is preferredto employ at least 30 percent diethylphosphite in the solvent mixture. It has been found-that solventmixtures containing 1 to percent of N,N-dimethylamide and 30 to 99percent diethyl phosphite will dissolve polymers containing more thanpercent by weight of acrylonitrile and are useful in the practice of thepresent invention.

Suitable acrylontrile copolymers which may be dissolved in mixtures ofdiethyl phosphite and the N,N-dimethylamides are the 'copolymers of morethan 90 percent by weight of acrylonitrile and at least 0.2 percent ofanother polymerizable monomer, for example vinyl acetate and other vinylesters 'of monocarboxylic acids, methyl methacrylate' and other alkylesters of methacrylic acid, ethyl acrylate and other alkyl esters ofacrylic acid, methacrylonitrile, vinylidene chloride, ethyl maleate andother alkyl esters *of m'aleic acid, ethyl fumarate and other alkylesters of'fumaric acid, styrene and other vinyl substitutedaromaticcompounds, alpha-methyl styrene'and other isopropenyl aromatichydrocarbons, vinyl chloride and other vinyl halides, vinyl pyridine andother vinyl substituted heterocyclic amines, and other polymerizablemonomers capable of polymerization with acrylonitrile. The mixture ofdiethyl phosphite and the N,N-dimethylamideisalso a solvent forcopolymers of more than90 percent by weight of acrylonitrile andaplurality of other monomers, for example the copolymers ofacrylonitrile, methacrylonitrile and vinyl acetate as described andclaimed in copending application Serial No. 786,153, filed November 14,1947, by Reid G. Fordyce and George E. Ham. j

In the practice of this invention, as it is in the preparation ofallacrylonitrile fibers, the molecularweight of-the polymer is of criticalimportance; The polymer should have a molecular Weight in-excess of10,000 and preferably in excess of 25,000. These molecular weightsaredetermined by measuring the viscosity of the polymer when dissolvedin a suitable solvent, such as N,N-dimethylformamide, in the manner wellknown to the art. It is also very desirable to use, acrylonitrilecopolymers which are substantially uniform throughout with respect tothe chemical composition and physical structure.

polymerization procedures are employed in the preparation of thepolymer. The subdivided states of the polymers obtained by spraydryingthe emulsions, or by filtration and subsequent drying of the solidpolymerszenablethemto be.-

used directly. The finely divided polymer is mixed with the solventmixture in any type. of mixing device, such as a dough mixer or ahomogenizer. It is desirable to use a solution of as high aconcentration of polymer as possible; but the maximum concentration isdependent upon the molecular weightof the. polymer.

Toobtain fibers of optimum physical proper ties polymersof molecularweights'i-n excess'oi 25,000 are used, and when using" such polymersitis-only possible to dissolve from five to 35-percent in the solventmixture without: exceeding practicable viscosity values. Although as lowas. five percent of the polymer can be used in spinning operations, suchconcentrations are undesirable because they necessitate the removal andrecovery or" toomuch solvent from the extruded solution, therebyincreasing solvent recovery. cost and reducing spinning speeds by reasonof the longer periods required for coagulation. The concentration ofthe. polymer in the solution is preferably between seven and 25 percentbut will ultimately be determin d by con sidering the desired physicalproperties of the fiber andthe speed of spinning, which speed is afunction of the concentration and viscosity of the polymer solution. Theviscosity willdepend uponthe chemical compositionand. the molecularweight of the polymers. The. optimum proportions can best be determinedby selecting a uniform molecular weight polymer having good fiberforming properties and dissolving it in the smallest amount of themixture of diethyl phosphite and, either l T,Ndimethylformamide orN,N-dimethylacetamide nec ssary, to.form aviscous solution capable ofextrusionat convenient temperatures.

The fibers are spun by extruding the solution of, the acrylonitrilepolymer through an orifice, or aspinneret having a plurality oforificesinto a medium which removes the solvent. The volume of thesolution passed throughthespinneret perunit of timemust be constantinorderto pro.- duce a fiber. of uniform. size. This is best achieved byusing a positively driven gear pump constructed of corrosion resistantmetals, .such as stainless steel, and adapted to deliver a constant flowof solution regardless of minor changes in viscosity and regardlessoftheresistance offered by the spinneret. It is also. desirable to passthe solution. throughcne .or. more filters before reaching the spinneretin. order. to remove all possible traces of foreign matter andparticlesof incompletely dissolved polymer. Thepolymer solutionmay be deliveredto the gear pump by meansof pressure applied by anine-rt gas to theliquid. surface of the solution reservoir, which must. be heatedtomaintain the solution. fluid enough .to pass through the conduits. Thegear pump, filter. devices and conduitstdthe spinneret are preferablyheat insulated .and .-may be heated to maintain the body of solution inliquid state. The extruding operation should be conducted at elevatedtemperatures but far enough below the boiling point of the solvent toprevent bubbles or other irregularities in the fiber.

The medium into which the solution is extruded and which removes thesolvent may be either liquid or gaseous. The method involving the use ofliquids is known as wet spinning; and usually utilizes aqueous media,which may have salts or other water soluble compounds dissolved'therein.The spin bath may be any liquid which is .a non-solvent for theacrylonitrile polymer but which either dissolves the solvent mixtureor.converts it into a soluble compound. The solvent is leached out of thestream of polymer solution, which first becomes a viscous stream and"finally a solid filament. When a spinneret with .a plurality ofapertures is used the several streams of polymer converge and ultimatelyform asinglefiber. The spin bath must necessarily be of sufil'cientsizeto permit. the complete, or substant'ially complete, removal of thesolvent mixture. Obviously, the rapidity of extrusion will affect thesize of the spin bath, high speeds requiring much longer baths. Thetemperature of the bath also aliects the size, higher temperaturespermitting, more rapid diffusion of the solvent mixturefromthe fiber andenabling the use of shorter baths.

The wet spinning method of fiber preparation is particularly adaptableto the use of mixtures of..,diethyl. phosphite and eitherN,N-dimethylacetamide or N,N-dimethylformamide as the polymer. solvent.Because of'the slower rate of solution .of the solvent mixture in wateras compared to the. rate ofsolution of other known solvents, for.example undiluted N,N-dimethyliormamide and. undiluted.N,lf-dimc-th'-'lacetamide, it is possibleto obtain clear, transparentfibers, whereas the fibers prepared from solutions. in. conventionalsolvents on precipitation in .water are cloudy. andopaque. For thisreason it. isfrequently desirable. to use longer precipitaticn baths orspinning baths adapted to provide longer immersion periods.

Theuse. of mixtures of diethyl phosphite and the said N,N-dimethylamidesas solvents for acrylonitrile copolym rs is also adapted to dry spinningoperations, wherein air, steam, nitro genor; other .gas,,or mixtures ofgaseswhich are inert at thespin temperature, are used to remove thesolvent. This method operates at higher temperature; and the solventmixture is evaporated from the surface. of the fiber. The maximumtemperature to which thefiberscan be subjected is. approximately theboiling point of the solvent mixture, since evaporation within thebodyof. the fiber may cause bubbles or other defects. The fiber may beheated by convection' from the hot gaseous medium or by radiation..fromthe-walls of the spinning cell. Gen.- erally .a combination of bothconvection and radiation is. involved, but methods involving principallyradiation aregenerally more efficient and permit. the operation withthewall temperature considerably in..excess of the boiling point of the.solvent. mixture. The evaporation of the solvent mixture from the fibersurface and the speedofthe fiber prevent. the-development of atemperature exceeding .that to which the fiber is stable. to adecomposition. The. dry spinning method. is particularly usefulat highrates. of extrusionlnigeneralithametliods of.'both.wet and. dry

spinning commercially used are adaptable for spinning from solutions inmixtures of diethyl phosphite and N,N-dimethylacetamide or N,N-dimethylformamide, but special considerations may be involved due to thedifferent chemical nature of the solvent mixtures. Automatic machineryfor spinning continuously, drying the thread if necessary, and windingit on suitable spools may be modified With the teaching of thisspecification. As in the case of most synthetic fibers, the fibers ofacrylonitrile copolymers spun from the solvent mixture solutions may bestretched to develop optimum physical properties. If desired part of thenecessary stretching may be accomplished in the spinning medium bydrawing the fiber out of the bath at a rate gnore rapid than the rate ofextrusion.

Example I A copolymer of 95 percent by weight of acrylonitrile and fivepercent of vinyl acetate was prepared in an aqueous medium. This polymerwas dispersed to the extent of ten percent in a mixture of equal partsof diethyl phosphite and N,N-dimethylacetamide. Upon heating thedispersion to 100 C. a homogeneous solution was obtained which retainedits stability when cooled to room temperature.

' The same polymer was dispersed in a mixture of 25 percent by weight ofN,N-dimethylacetamide and '75 percent diethyl phosphite. The dispersionwas heated to 140 C. for a prolonged period but no evidence ofsolubility was observed.

Example II A copolymer of 99 percent by weight of acrylonitrile and onepercent of vinyl acetate was prepared by polymerization in an aqueousmedium. The polymer was dispersed in a mixture of 70 parts by weight ofN,N-dimethylacetamide and 6 30 parts of diethyl phosphite. Upon heatingthe dispersion to 100 C. a transparent solution was obtained, whichretained its homogenity after cooling to room temperature.

The same polymer was dispersed in a mixture of equal parts ofN,N-dimethylacetamide and diethyl phosphite. The dispersion was heatedto 140 C. for a prolonged period but no evidence of solubility wasobserved.

The invention is defined by the following claims.

I claim:

1. A fiber-spinning composition comprising a homogeneous misciblemixture of an acrylonitrile copolymer and a liquid dispersion mediumconsisting of 30 to 99 percent of diethyl phosphite and one to percentof a dimethylamide selected from the group consisting ofN,N-dimethylacetamide and N,Ndimethylformamide, the acrylonitrilecopolymer being a polymer of more than percent and up to 90+0.14multiplied by the percentage of dimethylamide in the liquid dispersionmedium, by weight of acrylonitrile and at least 0.2 percent of anotherpolymerizable monomer, said copolymer having a molecular weight of atleast 10,000.

2. A fiber-spinning composition as defined in claim 1 wherein thedimethylamicle is N,N-dimethylacetamide.

3. A fiber-spinning composition as defined in claim 1 wherein thedimethylamide is N,N-dl-' methylformamide.

4. A fiber-spinning composition as defined in claim 1 wherein thepolymerizable monomer is vinyl acetate.

5. A fiber-spinning composition as defined in claim 4 wherein thedimethylamide is N,N-dimethylacetamide.

6. A fiber-spinning composition as defined in claim 4.- wherein thedimethylamide is N,N-dimethylformamide.

GEORGE E. HAM.

No references cited.

1. A FIBER-SPINNING COMPOSITION COMPRISING A HOMOGENEOUS MISCIBLEMIXTURE OF AN ACRYLONITRILE COPOLYMER AND A LIQUID DISPERSION MEDIUMCONSISTING OF 30 TO 99 PERCENT OF DIETHYL PHOSPHITE AND ONE TO 70PERCENT OF A DIMETHYLAMIDE SELECTED FROM THE GROUP CONSISTING OFN,N-DIMETHYLACETAMIDE AND N,N-DIMETHYLFORMAMIDE, THE ACRYLONITRILECOPOLYMER BEING A POLYMER OF MORE THAN 90 PERCENT AND UP TO 90+0.14MULTIPLIED BY THE PERCENTAGE OF DIMETHYLAMIDE IN THE LIQUID DISPERSIONMEDIUM, BY WEIGHT OF ACRYLONITRILE AND AT LEAST 0.2 PERCENT OF ANOTHERPOLYMERIZABLE MONOMER, SAID COPOLYMER HAVING A MOLECULAR WEIGHT OF ATLEAST 10,000.